Embodiments of the invention relate to a wind turbine. Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox.
Gearboxes form one of the most maintenance-intensive components of the wind turbine. They need to be inspected regularly and do not always fulfill their expected service life; the gearbox or some of its parts sometimes need to be replaced prematurely. This is due to the high loads and fluctuating loads to which a gearbox may be subjected. Particularly, the bending loads on the blades, which may be transmitted through the rotor shaft to the gearbox are damaging.
Direct drive wind turbines such as known from e.g. WO 2005/103489, do not suffer from the problems related to the gearbox. However, since there is no speed increase, the generator shaft rotates very slowly. As a consequence, a large and relatively expensive generator is generally needed to be able to generate electricity in an effective way. Additionally, when bending loads and movements (and corresponding deformations) are transmitted through the rotor shaft to the generator, it may not be possible to control the air gap between generator rotor and generator stator. High bending loads can even cause structural damage to parts of the generator, e.g. its bearings. Replacement or repair of such generator parts may be very expensive due to the size and related cost of the generator and its components.
Also in the case of more integrated direct drive wind turbine designs, which lack a rotor shaft and which have a direct coupling between the hub or its blades and the generator's rotor (known from e.g. DE 10255745), the bending moments and deformations are directly transmitted from the hub to the rotor and/or the stator, making it more difficult to minimize air gap variations.
The cause of the transmission of the bending loads and deformations from the blades and hub to the generator lies in the wind turbine configuration. In most conventional wind turbines, the rotor hub is mounted on one end of the rotor shaft. The rotor shaft is rotatably mounted in a support structure within the nacelle on top of the wind turbine tower. The rotor thus forms an overhanging structure which transmits torque, but additionally transmits cyclical bending loads due to the loads on the blades and the weight of the hub and blades. These bending loads are (in the case of direct drive turbines) transmitted to the generator causing air gap variations.
WO 01/59296 discloses a direct drive wind turbine comprising a hub with a plurality of blades, the hub being rotatably mounted relative to an axle part. The hub of the turbine is connected to the generator rotor by means of a plurality of connecting members, which are torsion stiff but yielding to bending moments. The generator may have an elongated ring-shaped element upon which the bearings supporting the generator rotor are provided. In other embodiments, the bearings are directly provided upon the frame.
With this kind of configuration the loads due to the weight of hub and blades are transmitted more directly via the frame to the tower, whereas the rotor transmits mainly torque to the generator, thus substantially reducing (but not completely avoiding) undesired deformations in the generator. This may mean improvement with respect to other prior art wind turbines, but air gap instability can still be a problem. Bending loads that are transmitted into the frame can cause deformations within the frame. Since the deformations along the length of the frame may vary under bending, the displacements experienced by the bearings of the rotor may vary. Also, the displacements experienced by the stator may be different from the displacements experienced by the rotor, thus causing air gap instability and deformations within the generator.
DE 10 2004 030 929 discloses a wind turbine with a nacelle, a rotor that is attached to the nacelle, and a generator comprising a generator stator and a generator rotor, wherein the generator rotor is coupled to the rotor and the generator is arranged on the opposite side of the nacelle with respect to the rotor.
At least one disadvantage associated with this configuration is the occurrence of high bending loads in the wind turbine tower. In larger offshore wind turbines, the weight of a generator may be close to or equal to the weight of a rotor. The rotor, with its long blades, will have to be arranged a minimum distance away from the tower to avoid the blades hitting the tower. By arranging the generator even further away from the tower, the bending loads in the tower may be very high.